Light-weight metal containers, including food and beverage containers, and the tooling employed to manufacture and form the metal food and beverage containers are generally known in the art. Typically, it is desired to manufacture metal food and beverage containers with as little material and the highest rates of production as possible, while maintaining desired quality characteristics. In typical metal forming processes used in metal container manufacturing, the metal is sized through various stages of tooling, which forms the desired shape and size of the container. The process often starts cutting a circular metal “blank” out of a sheet of metal. Then the metal blank (typically aluminum) placed into a cupping press, where the blank is “drawn” or pulled up into the shape of a cup, which has a diameter of about 3.5 inches to 4.0 inches and a height of about 1.3 inches for traditional 12 oz. cans. The following steps and processes typically work the cup (e.g., in a bodymaker machine) through numerous ironing and redrawing operations. This process reduces the wall thicknesses of the starting material and elongates the container into the desired final container shape. Here, a sleeve holds the cup in place and a punch is lowered swiftly into the cup and redraws the cup to a diameter of about 2.6 inches and a height of 2.25 inches for traditional 12 oz. cans. The punch (also called a punch sleeve) then pushes the cup against multiple ironing rings (typically three, also called ironing die), which stretch and thin the cup walls such that the cup for a traditional 12 oz. can is 5 inches tall. This entire process (the redrawing and ironing) is done in one continuous punch stroke.
The typical process of ironing deforms the metal between an ironing die and a punch sleeve. The metal is deformed under ultra-high forming pressures and loads, where the metal climbs or elongates up the punch surface as the container wall thickness is reduced and the container height is increased. The friction of the tools in the ironing process often determines the resultant quality of the ironing process.
Punch tooling (punch and die) is often made of hardened steel or tungsten carbide. A die is located on the opposite side of the workpiece and supports the material around the perimeter of the container. There is a small amount of clearance between the punch and the die to prevent the punch from sticking in the die. The amount of clearance needed depends on the thickness of the workpiece (i.e., the metallic container), with thicker materials requiring more clearance, but the clearance is always less than the thickness of the workpiece.
Traditional treatments of the punch surface finish have been consistently monolithic applications to amend the surface finish in an effort to control friction and removability of the can from the tooling once ironed. The friction of the punch, the ironing load, and the speed of the punch often determines the defect rates. Such defects are known as tearoffs. “Tearoffs” are a primary defect observed in the normal ironing process, where metal containers that experience excessive tensile loads have a portion of the container which breaks away from the remainder of the cylindrical container. The punch finish has a direct effect on the tear off frequency. Traditional treatments of the punch surface include polished surfaces or crosshatched surfaces that are applied to the entire surface of the punch such that the punch has a single surface treatment. Punch surface texture supports and improves container “stripability,” i.e., removal from the punch and ironing die. Thus, punch surface finishes are used to facilitate and improve stripability and, as a result, reduce the defect rates of tear-offs during the manufacturing process.
Common treatments of the punch surface include scoring the sidewall of the punch sleeve in crosshatching patterns. The angle and the coarseness of the crosshatching patterns may vary from punch to punch.
All ductile materials stretch to some extent during punching, which often causes the punch to stick in the container. If this happens, the punch must be physically pulled back out of the container and the container must be pulled off of the punch. This process is known as “stripping” because you are stripping the can from the punch and the defect is called “removability.” Additionally, the finer the finish on the punch, the more difficult the stripability (i.e., the ability to easily remove the can from the punch sleeve without excessive damage).
A third defect common in metal container manufacturing is due to the interaction of the punch's normal surface treatments in relationship to ironing forces and the actual releasing of ironing pressure during discharge of metal from the ironing dies. These defects are often described as “ears,” “pinch points,” or “feathered edges.” Ears are ripples at the top of the metal container after the drawing and ironing process. Additionally, ears are elongated regions with an irregular area that causes unequal loading of the ironing die and the resultant defects multiply creating feathered edges, pulled ears, or pinch points.
The material of the punch is often a very durable material, such as carbides, ceramics, and/or tool steel. However, due to millions and millions of cycles these finished surfaces degrade over time due to friction. The degradations cause deformation inconsistencies and changes to the sidewall characteristics of the metal container. Numerous defects can occur and the container walls may contain defects that require removal from the production stream and ultimately reduce the efficiency of the production process.